Process for developing an electrostatic latent image



Nov. 29, 1955 R. B. LANDRIGAN ETAL 2,725,304

PROCESS FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE Filed Aug. 51, 1951 FIG7 W FIGS INVENTORS RICHARD B. LANDRIGAN DONALD L. FAUSER RAY E. TOM

inn 9 Si ATTORNEY United States Patent PROCESS FOR DEVELOPING AN ELECTRGSTATIC LATENT IMAGE Richard B. Landrigau and Ray E. Tom, Columbus, and Donald L. Fauser, Cincinnati, Ohio, assigncrs, by mesne assignments, to The Haloid Company, Rochester, N. Y., a corporation of New York Application August 31, 1951, Serial No. 244,556

8 Claims. (Cl. 117i7.5)

This invention relates in general to xerography or electrophotography andiin particular to a method and apparatus for development of an electrostatic latent image.

In the process now' known as xerography an electrostatic latentimage is formed on a suitable insulating surface, such as, for: example, a photoconductive insulating layer on a conductive backing, and this electrostatic latent image is then developed by suitable means to yield a visible image which may be transferred, fixed, or otherwise used or employed. Perhaps the most critical step in the entire xerographic process, and the point where there is greatest room for improvement is this development step wherein the electrostatic latent image is transformed into a visible image. Thus, where the fine gradations of continuous tone photography are brought into the xerographic process, it has been found that prior means and methods for development are not capable of achieving the results otherwise realized by xerography, and that methods and means for creation of the electrostatic latent. image, for transfer of the visible image, and the like, are capable of higher quality production than has heretofore been possible in the development step.

Now, in accordance with the present invention, method and apparatus are provided for substantially improved development of an electrostatic latent image, whereby the image may be reproduced in a faithful and pleasing rendition of tone, density, continuity, and the like, improved resolution, andgenerally finer quality.

In general, the apparatus according to the invention comprises a cloud generating: means and cloud charging means whereby a cloud of powder, smoke, or the like, is first generated or dispersed and then given an electrostatic charge of substantially homogeneous polarity; and, in .a subsequent position a development zone in the form of anextremely thin space between an electrostatic latent image bearing member and a control electrode into which the charged powder cloud is introduced and caused to deposit on-the image bearing member. A duct or expansion zone is preferably interposed between the charging means and the development zone to direct the charged cloud into thedevelopment space.

It is, therefore, an object of the invention to provide apparatus and equipment to transform an electrostatic latent image into a useful and valuable visible image of improved quality.

It is another object of the invention to provide new and improved xerographic development apparatus and to provide an improved process for xerographic development.

It is a further object of the invention to provide a xerographic development process particularly for dense powder deposition at low image potentials wherein an electrostatic latent image is developed by a cloud of electrostatic particles, and to provide apparatus for carr ing out the process.

Additionalobjects of the invention will, in part, be

'ice

obvious to those skilled in the art and will, in part, be apparent from the following specifications and from the drawings in which:

Figure l is a diagrammatic side elevation in cross section of apparatus and equipment according to one embodiment of the invention;

Figure 2 is a diagrammatic view of a charging unit according to Figure 1, illustrating the electrical operation thereof;

Figure 3 is an enlarged fragmentary front elevation of the development assembly in Figure 1 taken in section along line 33;

Figure 4 is a similar view of a modified form of a development assembly;

Figure 5 is an isometric view of a development assembly according to another embodiment of the inven tion;

Figure 6 is a side elevation in section of a charging unit according to another embodiment of the invention;

Figure 7 is a side elevation in section of a developing assembly according to a still further embodiment of the invention;

Figure 8 is a diagrammatic cross section of an imagebearing member, illustrating fields of forces therearound; and

Figure 9 is a similar view of an image-bearing member with a development electrode positioned therebelow.

According to the process of the present invention a cloud of electroscopic powder particles is generated in a cloud-forming device, is passed through a charging zone within or optionally thence into an expansion zone, and then is passed into a developing zone adjacent to an electrostatic latent image. In this developing zone the charged particles are caused to deposit selectively on the electrostatic latent image surface by virtue of the electrical charges of the latent image and of the electroscopic powder, these selectively deposited electroscopic particles thereby forming a visible or developed image corresponding faithfully to the electrostatic latent image. This visible powder image then is used by further steps of xerographic processes, for example, by transfer, fixing, or other means to yield a xerographic print or to yield other useful forms of the xerographic image.

As shown in the figures in detail, this invention contemplates new development apparatus and methods of operation thereof. The apparatus shown in Figure 1 comprises generally a cloud generating unit, a cloud charging unit 12, and a development box 13, having a developing zone as defined by a development electrode 14 spaced extremely closely adjacent to an electrostatic image-bearing member 15. The cloud generating unit 11 is a closed container or powder reservoir, such as, for example, a box, can, bottle, jar, or the like, having sealed bottom and sides, and having a cover member 17 thereon. Within the chamber is a loose mass of electroscopic powder 18 comprising finely-divided particulate material capable of being dispersed in air and capable of accepting an electrostatic charge on its individual particles including, for example, charcoal, anthracite, pigmented resins, pigmented and dyed cellulosic derivatives and other finely divided powder materials. Extending through cover 17 and below the surface the powder mass is an air inlet tube 20, which at its lower end terminates in a ring 21 having a plurality of holes or openings 22 directed upwardly and optionally at an angle from the vertical. it is contemplated that some of these openings may be directed vertically, while others may be directed inwardly, and still others directed outwardly, whereby a stream of air introduced through the tube 20 will be widely scattered into the mass of powder. The air inlet tube 2 3 is connected outside the chamber to a suitable air supply source 24,

which may be an atomizer bulb, an air pump, compressed air tank, a carbon dioxide capsule, or other gas source.

A powder cloud exit tube 25 passes through cover 17 of the chamber and terminates optionally in nozzle 25:: within the chamber at a point above the powder mass. The opposite end of this tube is operatively connected to the powder charging unit 12, for example, by means of adapter 26. The powder charging unit 12, according to this embodiment of the invention, is a cloud charging zone, such-as, for example, a short tubular insulating member secured to the side of the box 13 by means of screws 28 through flange 29. Projecting through the walls of the charging unit are a plurality of electrodes 30 and 31, at least one of which (designated electrode 30) is electrically grounded or connected to a low potential source, and at least one of which (designated electrode 31) is connected to a high potential source. The structure of this charging unit is shown in greater detail in Figure 2, wherein the electrodes are illustrated, and wherein it is indicated that the high potential or charging electrodes 31 are conductively joined to a high potential source through resistors 32. These resistors may be of the ohmic type or preferably of the neon tube type, in either case serving the purpose of limiting or preventing arcing in the charging unit.

Referring again to Figure l, the powder charging unit 12 is mounted on walled chamber or development box 13, adjacent to and operating through inlet opening 32. This development box 13 comprises an enclosed space having side and bottom walls, and a top wall which optionally may be a counterelectrode or development electrode 14 forming one face of the development zone. Adjacent to one end of this development electrode 14 is an inlet slit 34 to the development zone. This slit, optionally, is spaced from the development box inlet opening 32, in such a manner that material passing through the box 13 must travel a substantial distance within the box.. In addition, slit 34 is optionally so directed that material passing through the box 13 may undergo reversal or change in direction in passing through the box. The plate at slit 34 preferably is so mounted that its lower or operating surface is substantially flush with the lower surface of top wall 16 of box assembly 13, whereby turbulence at this point is decreased and whereby the cloud enters the development space without substantial deviation from its direction of flow parallel with the imagebearing member.

On the upper edge of the boxv 13 and passing closely above development electrode 14 is a support means which receives an electrostatic latent image-bearing member, as illustrated in Figures 3 and 4. In Figure 3 the upper extensions of the rear and side walls terminate in shoulders 35, on which may be placed an image-bearing plate 15 in a position closely adjacent to the surface of development electrode 14. The development electrode 14 and the image-bearing member 15 are closely adjacent and spaced across a thin development zone or space according to the specific embodiment in Figure 1, this space being slightly tapered. The direction of the taper may be in either direction, but is illustrated as being such that the two members are more widely spaced near development inlet slit 34, and are most closely together at their opposite ends where they terminate in a narrow development exit slit 36. In actual measurements, the spacing at its widest is such that the space between the development electrode 14 and the image-bearing member 15 is at all points less than about one-eighth inch, generally in the neighborhood of about inch at inlet slit 34 and tapering to a substantially narrower space adjacent to the development exit slit, this exit slit being generally about one-third the width of inlet slit 34. It is particularly important that this development space he as thin as is consistent with its ability to accommodate a flow of powder cloud through the space so that the field of force from the electrostatic image is to the maximum extent drawn externally from 4 the image-bearing member as described hereinafter with reference to Figures 8 and 9.

In use and operation the apparatus disclosed herein is described with particular reference to Figure 1, it being understood that the operation of devices and apparatus as shown in the other figures will be obvious from this description. A powder mass 18 is first placed in the powder generating unit 11, optionally about enough to half fill the chamber, and preferably in an amount to maintain a powder level at least about 1'' above openings 22 in inlet tube 20. The cover 17 of the generating chamber is then tightly secured, the connections to and from the powder charging unit are fastened, and an electrostatic latent image-bearing member 15 is placed on the developing unit with its image-bearing surface facing development electrode 14. The powder charging unit is then energized, for example, by connecting charging electrodes 31 to a high voltage source, such asa direct current source of about 7,000 volts. The initial high voltage may be varied if desired, but is in a range to create a corona discharge between electrodes 31 and the ground electrode 30. In an actual electrical system used here the corona discharge electrode 31 may, of course, be the grounded electrode and the opposite electrode 30 maybe the high potential electrode. In fact, it is obvious that the requirement is merely that the electrodes be at highly different potentials such that a corona discharge results between them. For simplicity of description, however, the opposite electrode is arbitrarily referred to as a ground electrode, this not being intended as a limitation of its function. When the charging has been started and the corona discharge is occurring, air source 24 is operated to pass a stream of air through inlet tube 24) into powder mass 13, whereupon the powder is gradually dispersed to form a powder cloud in the free space in the powder generating chamber. This powder cloud is carried through tube 25 to the charging unit 12 wherein the corona discharge operates to deposit an electric charge on the surface of the individual particles. When high voltage electrodes 31 are connected to a positive polarity high voltage source a positive polarity corona discharge occurs and the individual particles of the powder cloud are positively charged. When these same electrodes are connected to a negative polarity high voltage source a negative discharge occurs and the individual powder particles are negatively charged.

The cloud of electrostatically charged powder particles then enters box 13 wherein several phenomena are believed to occur. In the first place, it is believed that momentum and convection currents cause some of these particles to approach the walls of box 13 wherein induced electric charges cause them to adhere. This phenomenon may be emphasized, if desired, by biasing one or more of the walls of this box at a desired potential with respect to ground to selectively attract particles of a desired or undesired charge. As another phenomenon within this box, it is believed that a space charge exists within the cloud, and that the presence of a large quantity of like charged particles causes mutual repulsion of the particles. The result of these and other phenomena which are not fully understood is that a' relatively uniformly-charged powder cloud emerges from the space within this box and passes through development inlet slit 34 into the development zone.

It is to be recognized that the various parts and components such as, particularly, the powder generating means, the powder charging means, and the development zone, may theoretically be positioned in space and perform their functions independently of external walls, supports and the like. In such an arrangement the purpose of the expansion box is to promote powder cloud homogeneity and to guide the charged powder into the development zone. However, in practical operation it is advisable to construct the entire development, equipment and apparatus as a structural unit and to protect it from external disturbance, in which case the box 13 comes into existence partly as a collecting box and as a housing or support for various of the functioning members.

Under ideal operation the powder cloud generating means is intended to generate a cloud of powder particles free from agglomerates or out-sized particles, and the charging means operates to impart to all of these particles a charge of like polarity and substantially uniform potential. It would, of course, be highly desirable to generate such a uniform charged cloud but practical operating conditions are such that the goal of uniformity is not fully achieved. It is in the case of a non-uniform cloud that the box serves to accomplish the purposes set forth above whereby the non-uniformity is reduced through separation or collection of out-sized particles or agglomerates and of particles having undesired charge potentials. It is apparent, therefore, that the box may well have a practical rather than theoretical purpose and may desirably be eliminated from the apparatus as techniques and means within the scope of the invention are improved. in fact, a large forward step in this direction has been accomplished in the mechanism and device shown in Figure 5 wherein the function of the box or zone becomes more in the nature of a guide zone and its collecting function is suppressed to a near minimum.

The development electrode 14 positioned close to the electrostatic image-bearing member 15 is at a desired potential during this development process, optionally being connected to a ground potential or a potential very near to ground potential. The presence of the closely adjacent development electrode 14 and electrostatic image-bearing member 15 creates an electric field across the space between these two electric members, which electric field faithfully represents the electrostatic image on member 15. Charged powder particles entering this space through slit 34 and passing through the exit slit 36 are subjected to the forces of the electric field. As these particles pass through this electric field they are, therefore, deflected from their course and caused to deposit.

In one embodiment of the invention the corona discharge from electrodes 31 and the electrostatic latent image on member 15 are of opposite polarity, whereby the charged powder particles are caused to deposit on the electrostatic image-bearing member in an arrang ment exactly corresponding to the electrostatic image originally thereon, to form a positive photographic reproduction. According to another embodiment of this invention the corona discharge from electrodes 31 and the electrostatic latent image on member 15 are of like polarity, whereby the particles are repelled from the charged source of the electrostatic latent image depositing either on the uncharged potential of the image-bearing surface to yield a negative or reversed image, or on the development electrode to yield thereon a positive image corresponding to the original electrostatic latent image. According to either of these embodiments of this invention an electrostatic latent image is developed into a visible and useful image of electroscopic powder.

Under certain circumstances, there is a tendency for the charged particles to deposit more rapidly and apparently more thickly in the areas nearer the development inlet slit. There is some reason to believe that this tendency may be overcome by having the development electrode and image-bearing surface spaced further apart near the entrance end and closer together near the exit end of the development zone; control of conditions such as charging potentials, cloud velocity and the like also afford satisfactory means of overcoming the problem. In the particular embodiment in Figure 1, this spacing is preferably in the form of a taper or incline of constant angle, whereby a uniform charge on the image-bearing surface causes a gradually increasing field between the members. Thus, a charged particle passing through this zone encounters a gradually increasing force as it continues .on its path and is, therefore, more strongly pro pelled toward deposition as it nears the far end of the development zone.

In Figure 4 is shown an alternative structure for the support of the image-bearing surface. According to this structure ledges 38 extend inwardly just below the top edge of side walls 39 of box 3.3. in this case the imagebearing surface 15 rests on these ledges and is cradled between Walls 39 in a fixed position. These ledges are positioned to support the image-bearing member 15 in the closely spaced tapered relationship with development electrode as described in connection with the previous figures.

in Figure 5 is illustrated a modified development unit according to another embodiment of the invention. This unit comprises generally a tapered member, such as, for example, a frustro-conical shell 41 about /2 inch at its smaller end and about 1 inch at its larger end and about 4 inches long. The shell is closed at its smaller end 42 and has an inlet opening 43 at its larger end, this opening being analogous to inlet opening 32 shown in Figure 1. This inlet opening is adapted to receive a powder charging unit, such as, for example, the powder charging unit of Figure 2. Mounted along and extending from one side of shell 41 is a supporting frame comprising development electrode 14 having side members or walls 44 extending upwardly therefrom. On these side members are ledges 45 adapted to receive and support an image-bearing surface T'nese side walls and ledges are of gradually tapered height, being slightly higher adjacent to the shell 41, and slightly lower at the end removed therefrom, the space between an image-bearing surface thereon and development electrode here again being tapered as illustrated in Figure 1. According to one form of the invention the heights from the tops of the walls to supporting ledges are less than fis inch and may be about inch at the shell, optionally tapering to about /3 that height at the end away from the shell, the space between electrode 14 and member 15 thereby being tapered between those widths. According to another embodiment, the spacing between electrode 1 and member 15 is uniform, being maintained as thin as is consistent with fiow of the powder cloud therethrough, generally being less than /8 inch and preferably about /y) inch. Along the side of shell 41 directly adjacent to development electrode 14 and positioned to open into t.e development space or zone between this electrode and the image-bearing surface is a development inlet slit :3 analogous in operation to the same slit in Figure l, the edges of this slit preferably being flush with electrode 14 and ledges 45.

In Figure 6 is illustrated a modified powder charging unit comprising a shell or tube 43 having a flange 49, whereby it is adapted to be connected to the development unit. A double-pointed charging needle 5i is mounted within the tube parallel with the axis of the tube, and optionally coincident with the axis. A conductive mounting support 51 extends from needle 5% through an insulating plug 52 in the wall of tube 48 terminating in an electric terminal 53 outside the tube whereby the n edle is adapted to be connected to a high voltage source and is analogous to electrode 31 in igure 2. The body of the tube 4'5 is adapted to be electrically grounded, whereby it is analogous to electrode 30 in Figure 2. It will be observed in this structure that at least both points of needle 50 and to a limited extent also the body of the needle generate a corona discharge, this double corona resulting in more uniform charging of the powder cloud.

The operation of the apparatus and equipment illustrated in Figures 5 and 6 substantially corresponds with the operation just described. in the developing unit shown in Figure 5 the collecting chamber is substantially reduced in size and its functions reduced, although, of course, these functions are not believed to be completely eliminated. The operation of this device when employed in the process is to produce a useful and valuable powder image faithfully reproducing an electrostatic latent image.

Similarly, the operation of the charging act shown in Figure 6 is similar to the operation of the charging unit shown in Figures 1 and 2. In this case, however, a corona discharge is generated to each point of charging electrode 50, whereupon a more uniform charge is believed to be imparted to powder particles passed therethrough.

Figure 7 illustrates a developing unit similar to that illustrated in Figure 1, comprising a box 13 having a development electrode 14 and adapted to receive an image-bearing member 15, all as described and illustrated in Figure 1. In the device shown in Figure 7 thedevelopment zone between electrode 14 and imagebearing member 15 is of substantially uniform thickness generally less than about one-quarter inch, and optionally about 0.01 to 0.025 inch. Under certain circumstances the development or deposition within the development zone does not vary from end to end and for these circumstances a uniform space between electrode 14 and member 15 creates a uniform electric field. Within the box 13 is mounted corona discharge electrode 31, whereby in this embodiment of the invention, and optionally also in other embodiments, the developing box includes the functions of the charging unit. The electrode operates to produce a discharge to the walls of the development box which, preferably, is grounded, and the resulting discharge operates to charge the powder particles within the development box. The form of the corona discharge electrode may be varied for special purposes and effects such as avoiding or limiting clogging or blocking of the electrode by deposition of charged powder thereon. In general, the corona discharge electrode will be a fine needle or fine wire disposed near to but spaced from another electrode maintained at a potential difference sufficient to support a corona or glow discharge therebetween.

In the drawings and in the description of the drawings, the electrical insulating or conductive properties of the structural and operating members have not generally been specified because interesting variations in the apparatus and method may be made through proper selection and co-ordination of these properties. When this invention is employed in an electrophotographic process, the usual image-bearing member will be a conductive backing such as a metallic plate bearing a photoconductive insulating layer on its surface, this layer being the portion carrying the electrostatic latent image. It is, therefore, possible to construct the development box entirely from conductive materials, all of which are electrically grounded. When the image-bearing member or plate is positioned on the box, all the members and parts will be at zero or ground potential except the insulating layer. As a variation, the developing box assembly may be conductive and optionally either grounded or biased at a desired potential, and the mounting supports for the image-bearing member may be constructed of an insulating material. In this manner, the developing box as a whole may be maintained at one potential, the backing material for the image-bearing member at a different potential and the image surface itself at a potential pattern determined by the nature of the electro static imagethereon. As a further variation, the development electrode may be insulated from the remainder of the developing box and thus biased at any desired potential, either the same as one of the other potentials or a still different potential, and this potential may, if desired, be varied during the process. As a further variation within the scope of the invention, it will be observed that interesting results may be achieved when at least one and optionally all the inner wall faces of the development box are insulators or insulator coated, in which case these walls will develop an electrostatic charge during the development operation. As a general procedure, however, it has been found adequate to have all parts electrically conductive except for insulators for the corona discharge electrodes 31- or 50 to prevent direct short circuits therefrom.

In co-ordination with the structures, it is preferred in one embodiment of the invention to have the development electrode at nearly the same potential as the image area of lowest potential, this usually being about the same as the potential of the conductive backing member in the case of a xerographic member. A slight variation contemplates a small potential difierence between these members, say in the order of about a six volt applied potential on the development electrode, this potential being of the same polarity as the electrostatic latent image. Under either of these conditions, the charged cloud passed therebetween may be of polarity opposite to image polarity, whereupon the cloud particles are attracted almost exclusively to the electrostatic latent image surface. As a corollary to this, and as described hereinbefore, the cloud particles may also be charged to like polarity with the electrostatic image, whereupon the field of force existing between the two members causes the particles to be deposited almost exclusively on the development electrode in a pattern reproducing the'electrostatic latent image. In the former case it is believed that the electrostatic image is neutralized by the charge on the deposited particles; in the latter case it is apparent that the image charge need not be neutralized and the electrostatic image may be again employed as a master for production of another image of electroscopic particles.

While it is not desired to limit any portion of this invention to a specific theory of operation, there is illustrated in Figures 8 and 9 an explanation of the operation of the development electrode 14, which explanation fits known facts. As shown in Figure 8 an image-bearing member having a conductive backing 55 and an insulating layer 56 thereon is charged with an electrostatic pattern or latent image as indicated by the plus signs to represent positive charges at the surface thereof. The electric field as illustrated by lines of force 57 is largely between the surface of the member and backing member 55. Only at the edges of a charged area do substantial amounts of the field of force project above the surface of the image-bearing member. However, when, as in Figure 9, the development electrode 14 is near the surface of the image-bearing member, a substantial proportion of the lines of force are external to the member and the field of force exists above the member as a faithful representation of the original electrostatic latent image. Thus, this external field of force, as applied by the development electrode (either grounded or biased at a desired potential) makes the electrostatic pattern on the member capable of improved development. Charged particles in this field are subjected to the field forces and are caused to deposit by virtue of these forces.

What is claimed is: a

1. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform insulating surface, said insulating surface being positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above said insulating surface by positioning a continuous conductive surface having a width and length at least equal to the area of said insulating surface to be developed and out of contact therewith and spaced uniformly therefrom no more than 4 said conductive surface being connected to a source of variable D. C. potential of the same polarity as the image, generating and charging a cloud of finely-divided powder particles and passing the charged cloud between said insulating surface and said conductive surface within said field of force.

2. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform insulating surface, said insulating surface being positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above said insulating surface by positioning a continuous conductive surface having a width and length at least equal to the area of said insulating surface to be developed and out of contact therewith and spaced uniformly therefrom no more than ,4 said conductive surface being at from ground potential to about 6 volts above the lowest potential of the image areas and of the same polarity, generating and charging a cloud of finely-divided powder particles and passing the charged cloud between said insulating surface and said conductive surface within said field of force.

3. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above the photoconductive insulating surface by positioning a continuous conductive surface having a width and length at least equal to the area of said photoconductive insulating surface to be developed and out of contact therewith and spaced uniformly therefrom no more than ,4 said conductive surface being connected to a source of variable D. C. potential of the same polarity as the image, generating and chargin a cloud of finely-divided powder particles and passing the charged cloud between said photoconductive insulating surface and said conductive surface within said field of force.

4-. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface positioned on a conductive backing member, said process comprising drawing the held of force of said image uniformly externally above the image-bearing surface by positioning a continuous conductive surface having a width and length at least equal to the areas of said image-bearing surface to be developed and out of contact therewith and spaced uniformly no more than from the image-bearing surface, said conductive surface being at from ground potential to about 6 volts above the lowest potential of the image areas and of the same polarity, generating and charging a cloud of finely-divided powder and passing the charged cloud between said image-bearing surface and said conductive surface within said field of force.

5. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above the image-bearing surface by positioning a continuous conductive surface having a width and length at least equal to the area of said image-bearing surface to be developed and out of contact therewith and spaced uniformly no more than ,4 from the image-bearing surface, said conductive surface being at from ground potential to about 6 volts above the lowest potential of the image areas and of the same polarity, generating and charging a cloud of finely-divided charcoal powder and passing the charged cloud between said image-bearing surface and said conductive surface within said field of force.

6. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface, said photoconductive insulating surface being positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above the imagebearing surface by positioning a continuous conductive surface having a width and length at least equal to the area of said image-bearing surface to be developed and out of contact therewith and spaced uniformly no more than from the image-bearing surface, placing said conducitve surface at ground potential, generating and charging a cloud of finely-divided charcoal powder and passing the charged cloud between said image-bearing surface and said conductive surface within said field of force.

7. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface, said photoconductive insulating surface being positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above the imagebearing surface by positioning a continuous conductive surface having a width and length at least equal to the area of said image-bearing surface to be developed and out of contact therewith and spaced uniformly no more than from the image-bearing surface, placing said conductive surface at the lowest charge of the image areas and of the same polarity, generating and charging a cloud of finely-divided charcoal powder and passing the charged cloud between said image-bearing surface and said conductive surface within said field of force.

8. A process for developing an electrostatic latent image comprising areas of varying charge on an uniform photoconductive insulating surface, said photoconductive insulating surface being positioned on a conductive backing member, said process comprising drawing the field of force of said image uniformly externally above the imagebearing surface by positioning a continuous conductive surface having a width and length at least equal to the area of said image-bearing surface to be developed and out of contact therewith and spaced uniformly no more than 4 from the image-bearing surface, placing said conductive surface at a potential about 6 volts above the lowest potential of the image areas and of the same polarity, generating and charging a cloud of finely-divided charcoal powder and passng the charged cloud between said image-bearing surface and said conductive surface within said field of force.

References Cited in the file of this patent UNITED STATES PATENTS 2,191,827 Benner et al. Feb. 27, 1940 2,221,776 Carlson Nov. 19, 1940 2,280,240 Kathe Apr. 21, 1942 2,281,638 Sukumlyn May 5, 1942 2,357,354 Penney Sept. 5, 1944 2,426,016 Gustin et al. Aug. 19, 1947 2,551,582 Carlson May 8, 1951 2,577,894 Jacob Dec. 11, 1951 2,600,129 Richards June 10, 1952 2,633,796 Pethick Apr. 7, 1953 FOREIGN PATENTS 605,979 Great Britain Aug. 4, 1948 

1. A PROCESS FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE COMPRISING AREAS OF VARYING CHARGE ON AN UNIFORM INSULATING SURFACE, SAID INSULATING SURFACE BEING POSITIONED ON A CONDUCTIVE BACKING MEMBER, SAID PROCESS COMPRISING DRAWING THE FIELD OF FORCE OF SAID IMAGE UNIFORMLY EXTREMALLY SAID INSULATING SURFACE BY POSITIONING A CONTINUOUS CONDUCTIVE SURFACE HAVING A WIDTH AND LENGTH AT LEAST EQUAL TO THE AREA OF SAID INSULATING SURFACE TO BE DEVELOPED AND OUT OF CONTACT THEREWITH AND SPACED UNIFORMLY THEREFROM NO MORE THAN 1/40", SAID CONDUCTIVE SURFACE BEING CONNECTED TO A SOURCE OF VARIABLE D.C. POTENTIAL OF THE SAME POLARITY AS THE IMAGE, GENERATING AND CHARGING A CLOUD OF FINELY-DIVIDED POWDER PARTICLES AND PASSING THE CHARGED CLOUD BETWEEN SAID INSULATING SURFACE AND SAID CONDUCTIVE SURFACE WITHIN SAID FIELD OF FORCE. 